Method for manufacturing paper with a constant filler content

ABSTRACT

The invention relates to a method of controlling the production of filler-containing paper. Filler and retention agent are both delivered to the paper manufacturing system in such a process of paper manufacture. The amount of filler in the paper is also measured, as is also the filler concentration or the total concentration in the white water or the stock. Filler is delivered in a quantity such that the system will be buffered with filler to a generally pre-determined concentration level (control value), which is followed via measuring processes. The continual addition of retention agent is based exclusively on the amount of tiller in the paper measured at that moment in time, such that when the measured amount of filler present is lower than a desired level, the addition of retention agent is increased, while when the measured amount or filler present is higher than a desired level, the addition of retention agent is reduced and the continual addition of filler is based, at least seen in the long term, exclusively on the concentration level of filler measured in the white water or the stock, and so that when the measured level is lower than the control value, the addition of filler is increased, while when the measured level is higher than the control value, the amount of filler added is decreased.

TECHNICAL FIELD

[0001] The present invention relates toga method for producing paperwith a constant filler content.

[0002] The invention is applicable in the production of any type ofpaper that contains a filler and a retention agent. The filler contentmay range from a very low level to a high level, for example from one ora few percent to up to 30%, of the total weight of the paper. Withrespect to weight, the retention agent content is much lower than thefiller content, normally only a fraction thereof, and generally has agiven relationship with respect to the filler content.

[0003] Paper is mainly comprised of pulp fibres. Pulp may be producedmechanically, chemimechanically and chemically. Lignocellulose material,including softwoods and hardwoods, is used as the starting material inthe production of pulp. A typical pulp furnish is comprised of differentpulp fibres in mixture. The pulps may be unbleached, semi-bleached andfully bleached, this latter pulp being the dominating pulp. Recycledfibres may constitute a base in the paper, either completely orpartially. The pulp may, at times, include an admixture of syntheticfibres. The paper may include one or more other ingredients in the formof paper chemicals. Examples of common such additives are starch,hydrophobising agent, nyancing colours and fluorescent whitening agent.Some papers are subjected to after treatment. Examples of aftertreatment are surface sizing, coating, and calendering.

[0004] Examples of types of paper that can include a filler are finepaper, i.e. non-coated and coated writing paper and printing paper,security paper, liner, label paper, formula paper and envelope paper.Wood containing printing paper, such as newsprint and magazine paper mayalso contain a filler.

BACKGROUND ART

[0005] A primary reason for providing paper with a filler is to improvecertain properties of the paper. One important property in this way isthe opacity of the paper, i.e. the non-transparency. Certain fillersimprove the brightness and/or the whiteness of the paper. One example ofsuch a filler is PCC (precipitated calcium carbonate), i.e. precipitatedcalcium carbonate. The filler can also improve the surface smoothness ofthe paper, resulting in improved printability. In addition, the majorityof fillers are significantly cheaper per unit weight (kilogram or tonne)than pulp fibres. This is particularly the case in relation to fullybleached chemical pulps. The admixture of filler thus leads to areduction in paper manufacturing costs. It is worth noting that there isa risk in using filler, and then particularly in large quantities, asthe strength of the paper is impaired to a greater or lesser extent incomparison with paper that includes no filler.

[0006] The manufacture of paper that contains a ‘filler commences’ withthe production of a thick pulp suspension. This suspension can beproduced in different ways. In the case of paper manufacture based ondry pulp in bale form, the pulp is slushed in water, usually white watertaken from the long circulation, such as to obtain a thick pulpsuspension. In the case of paper manufacture based on pulp in suspensionform that is delivered through a conduit to the paper mill from anadjacent pulp mill, the suspension is usually de-watered initially, forinstance from a consistency of about 2% to about 15%, so as to obtain acoherent pulp cake. The resultant water, free from pulp fibre, is sentback to the pulp mill through a conduit for renewed use as a vehicle forfeeding fresh pulp fibres to the paper mill. The pulp cake obtained inthe paper mill is broken-up and mixed with white water from the longcirculation, so as to obtain a thick pulp suspension.

[0007] The pulp fibres in the form of a thick pulp suspension arenormally subjected to a beating process prior to their further advancein the system. If the pulp furnish includes, for instance, two differentpulps, these pulps are usually each beaten separately before mixing thetwo pulp suspensions together.

[0008] Relatively large quantities of paper broke are obtained in thefollowing paper manufacturing process. There are several causes whypaper broke is obtained. A constantly dominating cause is because theouter edges of the advancing paper web are cut away as a matter ofroutine. Scrapping is another cause, i.e. the paper produced does notfulfil periodically the quality requirements placed on the paper. Athird cause can be that the advancing paper web breaks-off for somereason or other. Such broke paper is normally passed back to the papermanufacturing chain, after having been slushed in white water in brokepulpers. Because this starting material in the form of paper brokecontains a filler, the resultant thick pulp suspension will also containa filler. The amount of paper broke concerned may be as high as 40%, andeven higher, which is, in itself, a problem. However, a more difficultproblem in this connection is that the amount of paper broke normallyvaries with time. This means that the filler content of the incomingthick pulp suspension will also vary with time.

[0009] The thick pulp suspension is diluted with white water batch-wiseon its way to the short circulation and to the head box. One or morepaper chemicals can be delivered to the thick pulp suspension on suchsuspension diluting occasions. Significant dilution f the pulpsuspension with white water takes place at the beginning of the shortcirculation, for instance in the wire pit, so as to obtain a stock thathas a low solids substance content. Fresh filler can be delivered to thepulp suspension at several positions, for instance to the thick pulpsuspension or to the stock immediately downstream of the wire pit. Theretention agent can be delivered to the pulp suspension at describedpositions, and also later on in the short circulation, i.e. closer tothe head box.

[0010] The dominant part of the liquid phase-in the pulp suspension iscomprised of constantly circulating white water. However, a permanent ortemporary white water deficiency can be made up with fresh water.

[0011] The majority of fillers are in particle form that have a verysmall surface area (e.g. a diameter smaller than 10 μm) in relation tothe surface area or the size of pulp fibres (having a length of, e.g.,3000 μm and a-width of, e.g., 30 μm). There is a relatively small chanceof the filler fastening in the paper web by itself or beingspontaneously taken-up by the web. When forming the paper on the wirecloth, practically all pulp fibres will fasten on the cloth and form abed or network thereon. The number of holes in the network is determinedby many factors, among other things by the type of paper producingprocess applied precisely in the paper machine and also the weight perunit area or grammage of the paper produced. There is a directconnection between an increase in grammage and an increase in thethickness of the pulp fibre bed. It is natural that an increase in pulpfibres bed thickness will result in an increase in the amount of fillerthat is taken up. However, the spontaneous adsorption or retention offiller is insufficient to provide the desired content of filler in thepaper. It is therefore necessary to add one or more substances orchemicals that assist in incorporating filler in the pulp fibres bed andtherewith in the wet paper web as it leaves the wire and, e.g. is fedinto a press section of the paper machine. The wet paper web istransferred to an endless felt in conjunction therewith. This substanceor chemical is designated a retention agent. The use of a retentionagent results in comparatively more filler remaining in and accompanyingthe paper web, and comparatively less filler slipping through the pulpfibres bed and down through the wire cloth and into the wire traytogether with the drainage water or white water. Despite the use of aretention agent, and then even in large amounts, only a minor part ofthe filler present in the stock fed into the head box and thereafterspread on the wire cloth will fasten in the paper web, whereas a majorpart of the filler will accompany the drainage water as it passesthrough the paper web and the underlying wire cloth. This means that theamount of filler in the white water is still relatively high and is verylarge when seen in respect of the total amount of filler in the entiresystem (and then primarily in the large volume of white water thatcirculates in both the short circulation and the long circulation).

[0012] On the basis of the described circumstances, it will readily beseen that it is difficult to control the production of filler-containingpaper in a manner such that the final product, i.e. the finished paper,will constantly and persistently contain the desired filler content orfiller consistency, for instance expressed in a given percentage value.The buyer and the user of the paper are interested in that the qualityof the paper being always the same, and it is important in this respectthat the filler content of the paper is always the intended content andthat this filler content is achieved constantly from batch to batch.

[0013] In order to enable the manufacture of paper to be controlled inthe above-described respect, there have long been used measuringoperations that are carried out with the aid of a certain type ofmeasuring apparatus. One of these measuring operations involvesdetermining the filler content of the advancing paper web, normally atthe end of the paper machine, by means of a non-destructive measuringprocess, said filler content sometimes being referred to as the ashcontent. Another measuring operation involves determining the fillerconcentration of the white water either in the short circulation or indirect connection therewith. Alternatively, the filler concentration isdetermined together with the low quantity of pulp fibres that arealready present in the white water (total concentration). The twomeasuring processes are normally carried out intermittently, atintervals ranging from, e.g., only a few seconds to, e.g., thirtyseconds between respective measuring occasions.

[0014] In conventional control technology, both the addition ofretention agent and the addition of filler are varied in the course ofmaking the additions. The amount of retention agent added is based onprecisely the amount of filler measured in the white water, and theamount of filler added is based on the measured content of filler in thepaper web. It has been found that this control philosophy leads to arelatively significant variation in the filler content of the finishedpaper. Because the filler content of an accepted paper is only allowedto vary within a narrow range, the paper that must be scrapped becomesmuch too excessive. Moreover, as a result of this control philosophy,the switch from one filler content to another in the paper, for instancefrom 15 to 19 percent or vice versa, becomes unnecessarily extendedtime-wise and therewith necessitates unnecessarily the scrapping of muchof the paper. The earlier described problem caused by varying amounts offiller in the incoming thick pulp suspension is not overcome completelyby the described control philosophy. The attempt to correct a newlymeasured excessively low amount of filler in the finished paper with anincreased addition of filler to the thick pulp suspension for instance,or to the stock is doomed to failure to some extent, since the totalamount of liquid, chiefly white water, in the system is, as a whole,very large, meaning that the amount of circulating filler is also largeand also meaning that an instantaneous increase in the amount of filleradded to the system is unable to become quickly effective in respect ofan increase of the filler concentration in the circulating liquidsystem, which, in turn, would result in a higher quantity of fillerfastening in and being retained by the paper web. Such a system isextremely slow to control for these reasons.

[0015] Finnish Patent Application 97 4327 and its correspondingInternational (PCT) Patent Application WO 99/27182 describes, amongother things, a method which is alleged to afford advantages in the formof faster and more effective control of the paper properties in theshort circulation of the paper machine, in relation to known techniques.By paper properties is meant primarily the filler content of the paper.It would appear that the method concerned is based on the aforedescribedknown technique, which has been supplemented with an incompletelyexplained process in which both the continuous addition of filler andthe continuous addition of retention agent are both based on themeasured concentration of filler in the white water and the measuredfiller content or ash content (which is the term used) of the paper.

DISCLOSURE OF THE INVENTION

[0016] Technical Problem

[0017] As mentioned above, the technology used hitherto for controllingthe filler content in the manufacture of filler-containing paper resultsin excessively high variations in the filler contents obtained. Theapplication of such technology also results in excessively longchangeover times when changing from one filler content in the paper toanother. Both of these deficiencies result in the scrapping ofexcessively large quantities of finished paper.

[0018] The Solution

[0019] The present invention provides a solution to these problems andrelates to a method for producing paper with a constant filler content,wherein the method comprises

[0020] a) passing a thick pulp suspension, containing water, pulpfibres, normally filler originating from slushed paper broke andnormally diverse paper chemicals, in a direction towards the head box ofa paper machine;

[0021] b) adding water, normally white water, to the thick pulpsuspension on its way to the head box, so as to form a stock;

[0022] c) adding at least one filler to the thick pulp suspension and/orto the stock and/or to the water addition;

[0023] d) adding at least one retention agent to the thick pulpsuspension and/or to the stock and/or to the water addition;

[0024] e) spreading out a finally prepared stock over a wet apparatus,normally a wire section, via, the head box, so as to form a wet paperweb, and collecting the water drained from the web, designated whitewater, beneath the wet apparatus, and passing said white water back inthe paper manufacturing process for supplying said water to fresh thickpulp suspension, normally divided and included in two liquid-basedflows-of material, designated the short circulation and the longcirculation respectively;

[0025] f) causing the wet paper web to leave the wet apparatus andthereafter typically pressing and drying the web in at least one stageand optionally subsequently treating the web and/or collecting the webon rolls or converting the web into sheets; and

[0026] g) measuring the filler content of the paper in some position;

[0027] h) measuring the concentration of filler or the concentration offiller plus pulp fibres (the total concentration) in the white water orin the stock preferably in a position in the short circulation or indirect connection therewith, characterised by adding the filler in anamount such as to buffer the system with filler to a normallypredetermined concentration level (control value), said level beingevaluated via measurement process (h);

[0028] basing the continuing addition of filler exclusively on the levelof the filler concentration measured in the white water or in the stock,and increasing the amount of filler added when the measured level isbelow the control level and reducing the amount of filler added when themeasured level is higher than the control level at least when seen inthe long term, so that the white water system or the buffer system willalways make accessible sufficient filler to ensure that the paper webwill take-up the intended amount of filler; and

[0029] by basing the continuing addition of retention agent exclusivelyon the amount of filler measured in the paper (g) at that moment intime, and increasing the amount of retention agent added when themeasured amount of filler in the paper is lower than the level thatshall be held constant, and reducing the amount of retention agent addedwhen the level is higher than the level that shall be held constant,therewith resulting in fast correction of the filler content of thepaper back to the level that shall be held constant.

[0030] With regard to the filler, any known filler can be used. It isquite possible to use more than one filler. The filler or fillers can besupplied in one or more positions. It is usual that one filler is usedand that the total amount of filler is supplied to the stock in aposition at the beginning of the short circulation. There is nothing toprevent the filler addition being divided into two or more quantities,for instance into two part-quantities, of which one is delivered, to thethick pulp suspension and the other to the stock. It is optional whetheror not the additions of the two part-quantities are varied in the courseof the addition, or whether the addition of one part-quantity is fixedor constant and the addition of the other part-quantity is varied in thecourse of the addition sequence. Examples of fillers are kaolin clay,calcium carbonate (either in the form of substances that occurnaturally, such as limestone, marble and chalk or newly producedsubstances in the form of PCC), titanium dioxide and talcum.

[0031] The amount of filler charged to the system per unit of time isdependent of a number of factors, and a differentiation must be madebetween when filler is added in the start-up stage in the manufacture offiller-containing paper and when filler is added in a steady state. Ifpaper that contains a high filler content, e.g. 20%, is produced in asteady state, a large amount of filler is constantly taken from theliquid system or the white water, this filler entering and accompanyingthe wet paper web and because it is necessary to compensate the liquidsystem for the filler taken therefrom, at lest in the long term, it isnecessary to add a large amount of filler in said position or positions.When paper broke is used as part of the starting material which is anormal case (in addition to freshly supplied pulp fibres), already theincoming thick pulp suspension will contain a relatively large amount offiller. The amount of filler present will vary with the amount of paperbroke in the total amount of starting material and also on the amount offiller present in the paper broke concerned, for instance 10 versus 20%.The amount of filler that shall be added at a given point in time maybe, and often is, partly dependent on the circumstance just described.There is no absolute requirement for the addition of a given amount offiller on each addition occasion, in order for the invention tofunction. This is because of the presence of a filler buffer in thesystem, and the only absolute necessity is that the buffer system alwayshas available sufficient filler for the intended amount of filler to betaken-up in the paper web. This will be described also further on in thetext.

[0032] The filler is added by initially slurrying the filler in aliquid, for instance white water, and then delivering the liquidtogether with its filler content to the advancing pulp fibre suspensionor to the water in said possible positions, with the aid of a regulatoror with the aid of several regulators which operates/operate inaccordance with the described control philosophy. The regulator orregulators may be implemented in a computer program or may beconstructed mechanically, or may consist of electronic components.

[0033] Also regarding retention agent any known such agent whatsoevermaybe used. It is fully possible to limit the use to a single retentionagent that is delivered to the system at one or more positions. It maybe beneficial to use more than one retention agent, for instance tworetention agents. These agents may be added in one and the sameposition, although there is nothing to prevent each retention agent frombeing added to the system in a respective position. Both additives maybe varied in the course of making the additions, likewise that oneaddition amount can be kept constant, while the other addition amount isvaried time after time according to the need. Distinct from theposition, in which the filler is added to the system, it may bebeneficial to add at least a part of the requisite retention agentrelatively far forward in the short circulation, i.e. relatively closeto the head box. Examples of retention agent are inorganic retentionagents and synthetic water-soluble organic polymers.

[0034] Examples of inorganic retention agents are alun, bentonite clayand silica sols and diverse silicates. Examples of syntheticwater-soluble organic polymers are polyacryl amide, polyethylene amine,and polyamine. The polymers may be cationic, anionic and nonionicpolymers. The aforesaid paper chemical starch, which is available in anumber of different forms, is sometimes included in the retention agentgroup. It can at lest be maintained that the presence of starch in thesystem influences the retention of filler.

[0035] The amount of retention agent charged to the system per unit oftime is also dependent on several factors. Generally speaking, whenproducing paper that has a high filler content, more retention agentwill be consumed than when producing paper of low filler content. Onereason for this is because when producing filler-containing paper thereoccurs a spontaneous retention that is not influenced directly by thepresence of a retention agent. It can be mentioned in this connectionthat the spontaneous retention does not slavishly follow the presence offiller in the system and, e.g., the concentration of filler in the whitewater, even though this concentration normally increases with increasingconcentration of filler in the white water. As before mentioned, thisspontaneous retention is influenced by the grammage or weight per unitarea of the paper produced, and therewith also by the thickness of thepulp fibre bed or pulp fibre network forming the base in the paper web.The spontaneous retention is also influenced by the type of papermachine used. The amount of filler which is present in the paper andwhich has not been included via spontaneous retention is present due toand with the aid of the retention agent supplied to the system. Distinctfrom the case of filler, the system is not buffered with retention agentwhen this substance is added, but that an increase in the addition ofretention agent results almost instantaneously in an increase in theamount of filler incorporated in and fastening in said paper or paperweb. This is the reason of why, or a contributory factor of why, it ispossible to produce persistently a filler-containing paper with asubstantially constant filler content, as will be exemplified further onin the text. If it is chosen to set the amount of retention agentcharged to the system per unit of time in relation to the amount offiller charged per unit of time, it will be found that there is not anydirect relationship nor yet any relationship that varies uniformly.However, it can be said broadly that the weightwise addition ofretention agent lies within the range of some tenths of a percent toabout five percent of the addition of filler to the system. Thementioned relationship is primarily dependent on the filler content ofthe paper and on the type of paper.

[0036] The retention agent addition is effected by initially slurryingand/or dissolving the retention agent in liquid, for instance whitewater, and delivering said liquid with its retention agent content tothe advancing pulp fibre suspension or the water in said possiblepositions with the aid of a regulator or with the aid of severalregulators which operates/operate in accordance with the describedcontrol philosophy. The regulator or regulators may be implemented in acomputer program or constructed mechanically or comprised of electroniccomponents.

[0037] The apparatus that measures the filler content of the paper maybe placed anywhere adjacent the paper web, from the place at which apaper web is formed in the wire section to the place where the finishedpaper is rolled onto a roller or bobbin at the end of the paper machine.There are apparatus that are mounted in a fixed position adjacent theadvancing paper web and apparatus that traverse said web. The measuringapparatus may be placed very conveniently in a position where drying ofthe paper web is complete and where the web thus has a dry solidscontent in excess of 90%.

[0038] Any type of known measuring apparatus may be used. There isdescribed below a type of measuring apparatus that is used typically inthe production of filler-containing paper.

[0039] The apparatus is comprised of two parts, a transmitter partplaced beneath the paper web for instance, and a receiver part placedabove the web for instance. X-rays emitted by the transmitter part passthrough the paper web and up into the receiver part, where said rays areconverted to electric current of given voltage. Some of the X-rayspassing through the paper web collide with filler particles and areabsorbed thereby, resulting in the number of X-rays received by thereceiver differing from the number of X-rays emitted by the transmitter.The more filler particles present in the paper web, the more X-rays thatwill be absorbed and the weaker the electric current leaving thereceiver and measured as a weaker voltage. The measured voltagedifference is in relation to the difference in the amount of filler inthe paper, for instance given as a percentage of the grammage of thepaper. Examples of measuring apparatus that operate in accordance withthe described principles are Honeywell 2237-xx x-ray Ash Sensor and ABBAccuray, Smait2-Component and 3-Component Ash Sensors.

[0040] The described measuring apparatus that include a transmitter partand a receiver part can be fixedly mounted, i.e. such that measuring iseffected on solely one place of the advancing paper web. Alternatively,both the transmitter part and the receiver part may be mounted on ashuttle, such as to move synchronously with one another across theadvancing paper web and therewith measure across the fill width of theweb.

[0041] The filler content of the white water or the stock may also bemeasured by means of any known appropriate measuring apparatus. Adescription of a type of measuring apparatus used typically in themanufacture of filler-containing paper is described below.

[0042] The measuring apparatus includes, among other things, atransparent measuring cell. A given volume, e.g., white water is causedto flow through the cell per unit of time. Polarised laser light, thatis to say light of one and the same wavelength in one and the sameplane, is sent through the white water flow, which contains a largequantity of filler particles and a small quantity of pulp fibres, orrather fibre fragments. Part of the light rays impinge on the fillerparticles and the fibres/fibre fragments and rebound back and to theside in certain angular paths, these paths being dependent on the typeof material on which light rays impinge. Located immediately after thelight emitting location and in front of the measuring cell are lightsensitive detectors, which capture the light rebounding at differentangles. It is the light back scatter and the extinction at differentangles that is determined. The concentration of filler in white waterfor instance, can be determined in this way.

[0043] It is also possible to determine the total concentration of solidmaterial in, e.g., white water by means of a measuring operation. Thisis achieved by measuring the amount of polarised laser light thatsucceeds in passing through the white water unchanged, and by comparingthis quantity of light with the amount of polarised laser light emitted.The larger the amount of solid substances in the white water, the moretransmitted polarised laser light that is disturbed and becomesdepolarised.

[0044] KAJAANI RM-200, KAJAANI RM_(i) and BTG REG-5300 is one example ofmeasuring apparatus that functions in accordance with the abovedescribed principles.

[0045] Although the present invention finds its optimal application inpaper manufacturing processes in which some of the starting material isalways comprised of filler-containing paper broke, the invention canalso be applied with certain advantages in respect of the manufacture offiller-containing paper whose starting material contains no paper broke.

[0046] Advantages

[0047] One decisive advantage afforded by the inventive method is thatit results in surprisingly low deviations from the desired fillercontent of the paper. Consequently, the amount of paper that must bescrapped because of an error in filler content, is extremely low.

[0048] These low deviations in the filler content of the paper producedalso enables the control value to be set to a higher value than hashitherto been the case, when wishing to produce paper that has a high ora very high filler content. As before mentioned, high filler contentsresult in a reduction in the strength of the paper in relation to paperthat contains no filler. It is not the strength of the finished andconverted paper that is of primary interest, but the strength of thepaper web advancing in the paper machine. An excessively low webstrength can result in repeated breaks in the web, which, in turn,results in a high volume of paper broke and in low production of primepaper. When applying present-day control technology, the filler contentswings about a desired mean value quite significantly in bothdirections. When desiring a paper filler content that is only one ormore percentage points from the critical filler content at which theadvancing paper web can break at typical present-day very high machinespeeds, it is elected to place the control value on the desired fillercontent when practising conventional control technology, despiteeverything. This is done in order to ensure that as much as possible ofthe paper produced shall have a filler content that does not lie outsidethe accepted spread range. It should be noted in this respect that thespread downwards in filler content is not permitted to deviate more thanthe upward spread in filler content. The small variation in the fillercontent of paper produced in accordance with the present inventionresults in allowing the filler content control value to be laid in theupper half of the accepted spread range. The ability to controlpersistently the filler content so that said content will increase onaverage by only one percentage point has an immediate effect withrespect to paper manufacturing costs.

[0049] A similar advantage is also obtained at lower filler contents inthe paper which are not dangerous from a strength aspect i.e. thecontrol value may also then be laid in the upper half of the acceptedspread range which results persistently in a slightly higher fillercontent of the paper, therewith lowering paper manufacturing costs.

[0050] It has also been found that the inventive control method has amuch higher immediate effect than conventional control technology,leading to a short transition time in switching from one filler contentto another in the paper produced.

[0051] The low variation desired in the filler concentration in thewhite water and achieved in accordance with one preferred embodiment ofthe present invention, provides a smoother paper manufacturing sequenceand also results in fewer breakdowns in the paper manufacturing process.

DESCRIPTION OF THE DRAWINGS

[0052]FIG. 1 is a flowchart, which illustrates application of theinventive method in the manufacture of filler-containing paper.

[0053]FIG. 2 is a diagram illustrating in percent the filler content ofpaper produced in accordance with conventional technology and inaccordance with the invention.

[0054]FIG. 3 is a diagram illustrating the filler concentration in thewhite water in grams per litre in the manufacture of filler-containingpaper in accordance with conventional technology and in accordance withone preferred embodiment of the invention.

BEST EMB DIMENT

[0055] The inventive method will now be described partially in moredetail with reference to the flowchart of FIG. 1, and finally withreference to an exemplifying embodiment.

[0056]FIG. 1 is a schematic illustration of one embodiment of theinventive method.

[0057] A thick pulp suspension is fed into the short circulation 2through the conduit 1. The thick pulp suspension contains pulp fibres(whether or not one type of pulp fibre or several, e.g. two, types ofpulp fibres is included will depend on the type of paper to beproduced), water (predominantly white water) filler (originating fromthe paper broke slurry) and one or more paper chemicals. The thick pulpsuspension fed into the short circulation 2 through the conduit 1 mayhave a pulp concentration of 2 to 4%.

[0058] The thick pulp suspension is introduced in the conduit 3, whichcontains white water originating from a deaeration tank 4. The thickpulp suspension is therewith diluted and fed into the wire pit 5. Thepulp suspension is diluted further with white water in the pit, saidwater being passed from the wire tray 6 to the wire pit 5 through theconduit 7. This results in a stock. The thick pulp suspension deliveredthrough the conduit 1 is sometimes referred to as the stock, includingby certain persons skilled in this art. Although such language usage isnot wrong, we have chosen in this document to differentiate betweenthick pulp suspension and stock in order to be able to describe theinventive method in a simpler and more readily understood manner.

[0059] Fresh filler is fed to the stock in the form of an aqueousdispersion to the outlet of the wire pit 5, through the conduit 8. Theamount of filler added is determined primarily by the desired fillercontent of the finished paper. The method in which the addition offiller is regulated in detail will be explained further on in the text.Different types of filler have been exemplified in the aforegoing, andthe filler chosen in the individual case is dependent on severalfactors.

[0060] The stock is passed further through the conduit 10, by means ofthe pump 9. Because the filler is delivered close to the pump 9, thefiller will be effectively mixed with and distributed in the stock.Branch conduits 11 and 12 pass the stock to a respective battery 13 and14 of vortex cleaners or hydrocyclones. Accept pulp is passed throughthe branch conduits 15 and 16 and through the conduit 17 to theaforesaid deaeration tank 4. Reject is recovered and passed through theconduit 21 to a separate handling facility, which is not discussed here.The stock is delivered to the tank 4 through a large number of dipperconduits. As the name denotes, the stock is deaerated in said tank 4 andstock is passed from the tank in a substantially air-free state andcontaining a certain amount of white water (this latter being mentionedearlier) further along the system. A foam damping chemical can bedelivered to the stock upstream of position 4, with the intention oflimiting foaming of the stock.

[0061] The stock is fed to a screening operation by means of the feedpump 18, via the conduits 19 and 20. A first retention agent isdelivered to the stock in conduit 19 immediately upstream of the pump18, through the conduit 22. The retention agent may be slurried in ordissolved in white water. Each of the branch conduits 23 and 24 feed thestock to a respective screen 25 and 26. Accept pulp is fed to the headbox 30, through the branch conduits 27 and 28 and through the conduit29. Reject obtained in the screening operation is recovered and passedthrough the conduit 31 to a separate handling facility, which is notdescribed here. A second retention agent is delivered to the stock inconduit 29 immediately upstream of the head box 30, through the conduit32. This retention agent may be slurried or dissolved in white water.This results in an essentially final or finished stock.

[0062] The stock is distributed over a wire in a wire section 33, withthe aid of the head box 30. The solid substance concentration of thestock, essentially comprised of pulp fibres, ranges from 0.5 to 1.5percent in the described position. Concurrently with the formation of apaper web on the wire, a large amount of liquid or water is drained-offboth gravitationally and with the aid of suction boxes. This liquid orsaid water, designated white water, is collected in the wire tray 6.Part of the white water taken from the wire tray 6 to the wire pit 5through the conduit 7 is drawn-off through the conduit 34 and returnedto the head box 30 for final dilution of the stock inside the head box30 and in a particular part thereof.

[0063] The resultant, coherent paper web 35 is passed to a press section36 and thereafter to a pre-dryer 37 and then to an after-dryer 38,whereafter it is finally rolled-up on a reeling drum (tambour) 39.

[0064] The content of filler in the finished paper, for instance givenin a percentage of the weight of the paper, is determined intermittentlyby means of a measuring apparatus 40, which may be a traversing type inaccordance with what has earlier been described. The measurement signal,i.e. the measured filler content, is sent to a filler content regulator41, which sends a signal to a flow regulator 42 that controls the flowof retention agent t be supplied via the conduit 22. More specifically,the valve seated in the conduit 22 is controlled in a known manner toopen wider when desiring a higher flow of retention agent and to closeaccordingly such as to reduce the through-passage of retention agentwhen desiring a reduction in the flow of retention agent. The flowregulating system also includes a flow meter by means of which it can beensured that the desired amount of retention agent will actually flowthrough the conduit 22. In order to minimise disturbances in the fillercontent during a change in the production of the paper machine, meteringof the retention agent can be given a feed-forward signal so that itwill automatically follow the change in production. An increase inproduction requires an increase in the amount of retention agent meteredto the system. The feed-forward facility is designed so that a givenpercentage change in production will result in the same percentagechange in the amount of retention agent metered to the system. Thistakes place over and above the described control relating to themeasured filler content of the paper.

[0065] There is coupled to the conduit 34, through which white waterflows, an apparatus 43 for intermittently measuring the fillerconcentration and/or the total concentration in the white water. Atypical measuring apparatus includes a transparent measuring cellthrough which a very small volume of white water is caused to flow. Themanner in how measuring is effected has been described in more detailearlier. A signal which describes, e.g. the measured fillerconcentration in grams per litre of white water is sent from themeasuring apparatus 43 to the filler concentration regulator 44. Asignal is sent from the regulator 44 to a flow regulator 45, whichcontrols the flow of filler to be delivered to the system, via theconduit 8. This regulator 45 operates in a similar manner to theregulator 42 and also includes a flow meter in this case.

[0066] In order to minimise disturbances in the filler concentration ofthe white water during a change in the production of the paper machine,the filler flow can be given a forward-feed signal so that it willautomatically follow changes in filler requirement. Increased productionr an increase in the c ntrol value in respect of filler in the papergives, in the long run, a need to increase the amount of filler meteredto the system. By multiplying the production of the paper machine by thecontrol value for the filler content of the paper, there is obtained avalue for calculated filler consumption. The feed-forward coupling isdesigned so that a given percentage change in the calculated fillerconsumption will also give the aforedescribed adjustment in respect ofthe measured concentration of filler in the white water.

[0067] In the case of the described embodiment of the invention, onlyone filler is supplied (at position 8), whereas two retention agents aresupplied (at positions 22 and 32). With regard to the retention agentsupplied at position 32, which agent may consist of bentonite clay forinstance, the amount of agent supplied has been chosen to have a fixedvalue, i.e, one and the same flow of retention agent is supplied to oneand the same flow of stock The magnitude of this fixed charge ofretention agent will depend on a number of factors, such as on thedesired amount of filler in the finished paper and the amount of fillercharged to the system per unit of time, and also on the magnitude of theamount of supplementary retention agent charged to the system atposition 22. When using bentonite clay as retention agent, it has beenfound that an optimal effect is obtained when said agent is added to thesystem as close as possible to the head boy.

[0068] With regard to the retention agent in position 22, which agentmay, for instance, comprise a synthetic water-soluble organic polymer,the amount of agent charged varies in accordance with requirements. Ithas been found that in order to obtain a good effect with such aretention agent, the agent should be charged to the system immediatelyupstream of the feeder pump. 18, as shown in FIG. 1. Although it isfully possible to add the retention agent earlier in the flow directionwithin the short circulation, there is a risk that the retention agentwill then take several paths and be recycled, therewith causing theagent to lose electric charge and not being utilised optimally in thepaper forming process, i.e. in the wire section 33.

[0069] In accordance with the earlier described control philosophy, thevarying addition of retention agent in position 22 is effected in thefollowing way.

[0070] When wishing to produce a paper that includes a given filler in agiven quantity, for instance 21% it is known through experience that agiven approximate flow of filler must be delivered through the conduit8. It is also known through experience that in the current conditions,it is suitable to add a given retention agent to the system in a fixedamount, via the conduit 32. It is also known from experience what theapproximate. addition of said second retention agent shall be, via theconduit 22. When the paper manufacturing process is well underway, thefiller content of the finished paper is measured at short intervals inposition 40. If these measurements show that the filler content orconcentration of the paper is, for instance, 21.5% instead of 21.0%, thecontrol function is activated. The measured value is sent in signal formfrom position 40 to the filler content regulator 41, and said fillercontent regulator 41 sends to the retention agent flow regulator 42 asignal which indicates that the flow of retention agent shall bedecreased to a certain extent, because the measurement just taken showsthat the filler content of the paper is slightly too high. The reducedsupply of retention agent to the stock is quickly effective in reducingthe adsorption of filler in the paper web being formed on the wire,therewith obtaining the desired filler content of 21% in the paper. Ifthe measured filler content is lower than that desired, for instance20.5%, the flow of retention agent is increased through the conduit 22to a corresponding degree. The increase supply of retention agent to thestock quickly becomes effective in an increased adsorption of filler inthe paper web on its way being formed on the wire, therewith obtainingthe desired filler content of 21% in the paper.

[0071] In order to achieve the aforedescribed, the filler concentrationin the system, including in the white water, need not have a fixedrelationship with the amount of retention agent added to the system andthe content of filler in the paper produced, since it is also possibleto maintain a correct filler content in the paper when the continualaddition of filler over a longer period of time is excessively low andresults in a constant reduction in filler concentration in the whitewater. There is, of course, a lower limit for depletion of filler in thebuffer system.

[0072] The structure of the filler content regulator 41 is known to theart. A feedback regulator is normally used. The most common type ofregulator is designated PID regulator and operates exclusively on thebasis of “control error” e, and the following relationship prevailsbetween control error e and control signal u;$u = {K\lbrack {e + {T_{D}\frac{e}{t}} + {\frac{1}{T_{1}}{\int_{\quad}^{t}{{e(s)}\quad {s}}}}} \rbrack}$

[0073] The control signal is composed of three terms, where P denotesthe proportional term, which is proportional to the error, D denotes thederivative term, which is proportional to the derivative of the error,and 1 is the integral term, which is proportional to the derivative ofthe error. This is taught, for instance, in a booklet form Lund'sTeknisska Högskola entitled “Reglerteknik, en elementär introduktion”,written by Karl Johan Åström. The different terms are combinedadditively in the formula. A desired function is set in the regulator,by adjusting the three constants K, T₁ and T_(D). A number of differentmethods are available for adapting these constants to the process to beregulated. One usable method is designated the Lambda method.

[0074] As earlier mentioned, the flow of filler through the conduit 8 isessentially at least partially dependent of the filler content of thepaper produced in other words the amount of filler that is constantlyadsorbed by and incorporated in the paper web formed on the wire in thewire section 33.

[0075] The filler concentration of the white water is checked at givenintervals with the aid of the measuring apparatus 43. Normally, thedesired level of the filler concentration in the short circulation isone and the same for a given paper quality. This has to do with therunability of the paper machine. It has been found beneficial withrespect to the running of the paper machine to maintain the fillerconcentration in the system, including the filler concentration of thewhite water, constant over the passage of time. The control value may,for instance, be 4 grams per litre. If the measured value is 3.8 gramsper litre, this value is sent to the filler concentration regulator 44in signal form. This regulator sends, in turn, to the filler flowregulator 45 a signal to the effect that the flow of filler in theconduit should be increased, which is effected by opening the valve inthe conduit 8 connected to the regulator 45 still wider. The flowregulator system also includes a flow meter by means of which it isascertained whether or not the intended amount of flow actually flowsthrough the conduit 8. If it is found that the value measured is toohigh, for instance 4.2 grams per litre, the flow of filler through theconduit 8 is reduced to a corresponding degree.

[0076] The filler concentration regulator 44 is of a known kind and maybe of the same type as that earlier described, i.e. as the regulatorlocated in position 41. The control system constructed around theregulator 44 takes into account that the buffer system for the filler inthe short circulation, including all white water, is slow to adjust. Inother words, even though the flow of filler is greatly increased in acertain position, it will take a long time before the punctiformsignificant increase in filler will result in an increase in the fillerconcentration in the total, very large, volume of white water. Thecontrol program for the regulator 44 is generally similar to the controlprogram for the filler content regulator 41 described above.

[0077] As will be seen from the flowchart illustrating the production offiller-containing paper in accordance with FIG. 1, the long circulationbased on white water (for instance taken somewhere along the conduit 7)is not included, and neither are all the work-up stations for the thickpulp suspension delivered to the short circulation through the conduit1. This has been excluded for reasons of scope and clarity.

EXAMPLE 1

[0078] The inventive method has been tested in a paper machine of a kindthat coincided to a large extent with the flow chart according to FIG.1, for the production of filler containing fine paper. Comparisons weremade with conventional technology for the production of such paper;

[0079] A thick pulp suspension was fed through the conduit 1 at a flowrate of 16,500 litres per minute. The starting material for the thickpulp suspension was 60% fresh pulp delivered from an adjacent pulp mill,and 40% paper broke. In turn, the fresh pulp comprised 65% birchsulphate pulp having a brightness of 90% ISO, and 35% pine sulphate pulphaving a brightness of 90% ISO. Each of the two fresh pulps were refinedper se before being mixed in a mixing vessel, into which the slushedpaper broke was also fed. The paper broke had a filler content of about21.5%, and the filler comprised precipitated calcium carbonate (PCC).The incoming thick pulp suspension thus contained a significant amountof filler, which can be readily estimated. Stock starch was added to thethick pulp suspension on its way to the conduit 1.

[0080] Fresh filler in'the form of 52 percentage PCC was deliveredthrough the conduit 8 at an approximate flow rate of 90 litres perminute. The filler density was 770 grams per litre. Small quantities ofa number of colour tints were added at the same time. Additional paperchemicals, including fluorescent whitening agent, were added furtherforward in the short circulation.

[0081] A first retention agent in the form of a synthetic polymer havinga density of 4 g/l was delivered through the conduit 22. The flow rateof this retention agent was, on average, about 50 litres per minute.

[0082] A second retention agent in the form of bentonite clay having adensity of 35 grams per litre was delivered to the system via theconduit 32. The flow rate of this retention agent was fixed andconstituted 30 litres per minute throughout.

[0083] The stock leaving the head box 30 had a solid substance contentof 0.9-1.0%. The control value for the filler content of the finishedpaper was 21.5%, and the weight per unit area of the paper was 80 gramsper square metre. The machine speed was about 970 metres per minute,resulting in a production of about 30 tonnes of paper per hour. Thefinished paper had a moisture content of about 4.5%.

[0084] The paper was surface sized in a film press at a position late inthe paper manufacturing chain. The surface size was applied in an amountcorresponding to about 4 grams per square metre. Although no film presshas been shown in the flowchart of FIG. 1, the press was placedimmediately downstream of the pre-dryer 37 in the paper machineconcerned.

[0085]FIG. 2 illustrates the filler content of the finished paper overfour calendar days when using conventional technology in producingfiller-containing paper, and also the filler content of the, finishedpaper over a following four calendar-day period when using inventivetechnology in the production of filler-containing paper.

[0086] By conventional technology is meant, among other things, that thefiller content of the paper is measured in position 40 and also thefiller concentration in the white water at position 43. However, themeasured filler content of the paper is not used to control the additionof retention agent in position 22 but is used for controlling theaddition of filler at position 8. The control was carried out so that ifthe measured value of the filler content of the paper was higher thanthe desired value, i.e. the control value, the flow of filler wasreduced in position 8, whereas if the measured value was too low, theflow of filler was increased in position 8. Moreover, the flow ofretention agent in position 22 was controlled so that if the fillerconcentration in the white water, i.e. in position 43, was higher thanthe control value, the flow of retention agent was increased in position22, whereas if the measured value of the filler concentration was toolow, the flow of retention agent was reduced in position 22.

[0087] The filler content of the finished paper when applying theaforedescribed conventional control technology is shown to the left ofthe arrow in FIG. 2. As will be seen, the filler content varies greatlyaround the desired control values. The system has even reached a howlingin any occasion.

[0088] At the time marked with an arrow in FIG. 2, a departure from theaforedescribed conventional control technology was made, insofar as thesignal for the measured filler content of the paper at position 40 wassent to the filler content regulator 41, which, in turn, sent a signalto the retention agent flow regulator 42 in accordance with the FIG. 1illustration and in accordance with the inventive control technologydescribed above in detail. When applying the novel control technology,measuring of the filler concentration in the white water in position 43was released during the first calendar day from the automatic andcomputer controlled control system. Instead, metering of filler atposition 8 was effected manually by the operators during this calendarday.

[0089] It will be seen from FIG. 2 that a control value for the fillercontent in the finished paper of 21.5% was used over a period of about2.5 calendar days when practising the invention. The control value wasthen switched to 22.0%, which was followed by a short period in whichthe old control value was used, i.e. the value of 21.5%, and the testrun was terminated with a control value of 22%.

[0090] The superiority of the novel control technology over conventionalcontrol technology is clearly evident from FIG. 2. When applying thenovel technology, the variation in the filler content of the finishedpaper is reduced significantly in relation to the old and conventionaltechnology. The standard deviation in the filler content of manufacturedpaper has been calculated for one calendar day on each side of the arrowin FIG. 2 at a control value of 21.5%. In the case of the traditionalcontrol technology, the standard deviation was 0.95 and in the case ofthe inventive control technology the standard deviation was 0.14, inother words the variation in filler content of the paper was improvedalmost seven times when practising the inventive control technology.

[0091] The automated and computer controlled system for metering fillerin position 8 on the basis of the filler concentration measured in thewhite water in position 43 was activated after about one calendar day.How that works has been described in detail earlier.

[0092] As will be evident from FIG. 3, the control value was 4 grams perlitre both in respect of conventional control technology (to the left ofthe arrow) and with respect to a preferred embodiment of the inventivecontrol technology (to the right of the arrow). The variation around thecontrol value for the filler concentration also varies in a surprisinglysignificant manner in this case. It has been found that a low variationaround the control value for the filler concentration in the white wateris beneficial with respect to the drivability of the paper machineconcerned.

[0093] In the aforedescribed test run carried out in accordance with theinvention, data relating to the filler content of the paper was obtainedevery twenty seconds, while information relating to filler concentrationin the white water was obtained every four seconds. The use of preciselythese measuring intervals is in no way mandatory, but that the measuringintervals can be determined individually and are dependent on the typeof measuring apparatus used, among other things.

[0094] A study of FIGS. 2 and 3 will show that it is not absolutelynecessary to begin to control the addition of filler to the system onthe basis of the filler concentration measured in the white water inorder to obtain an essentially constant filler content in the paper dayafter day, although such a measure is preferred chiefly for otherreasons. In the described test run, control of the filler addition inposition 8 in accordance with a preferred embodiment of the inventionwas not commenced until after one calendar day. Despite this, the paperhad the correct filler content after only some ten minutes subsequent tostarting the test in accordance with the invention.

[0095] When the non-compulsory “second” control was started-up, theaddition of filler in position 8 was changed each time the measurementtaken in position 43 showed that it was appropriate to do so. In otherwords, a small change could be made to the flow of filler during longperiods each fourth second. It is in no way absolutely necessary to doso, since it is fully possible to make a relatively significant changein the flow of filler on the basis of a given measurement in position43, over a given period of time which experience has shown will resultin a general increase in the filler concentration of the entire systemafter a given, relatively long period of time, and check that themeasurements in position 43 follow a well-known pattern during said timeperiod. Thus, it is not a catastrophe if the flow of filler suddenlyceases in position 8 for some possible unintended reason, and that thesupply of filler is stopped for a limited period of time.

[0096] In principle, the conditions are different with respect to theaddition of retention agent in position 22 in this case, the commandgiven by the control system to increase the flow of retention agent isignored, the paper will obtain an excessively low filler content duringessentially this ignoring period.

[0097] It is mentioned in conclusion that there are found curves whichare similar to the curve shown in FIG. 2 and which confirm thatswitching of the filler content in manufactured paper from one level toanother can be effected much more rapidly with the inventive technologythan with conventional technology. This fact also contributes towardsminimising the volume of paper that need be scrapped. The fact thatpaper is still scrapped is, among other things, due to the fact thatquality parameters other than filler content can deviate from setmeasurement values. The aforesaid curves have not been included forreasons of space and scope.

1. A method for manufacturing paper with a constant filler content,comprising a) passing a thick pulp suspension containing water, pulpfibres, generally filler derived from slurrying paper broke, andgenerally diverse paper chemicals towards a head box in a paper machine;b) adding further water, normally white water, to the thick pulpsuspension on its way to the head box, so as to form a stock; c) addingat least one filler to the thick pulp suspension and/or the stock and/orsaid further water; d) adding at least one retention agent to the thickpulp suspension and/or to the stock and/or to said further water, e)spreading finally prepared stock on a wet apparatus, normally a wiresection, via the head box, such as to form a wet paper web, andcollecting resultant drainage water, designated white water, beneath thewet apparatus and passing said water back in the pap r productionprocess for delivery to fresh thick pulp suspension, normally dividedand included in two liquid-based flows of material designated the shortcirculation and the long circulation respectively; f) causing the wetpaper web to leave the wet apparatus and then generally pressing anddrying said web in at least one stage and subjecting said web optionallyto aftertreatment and/or collecting said web on rolls or converting theweb into sheets and g) measuring the filler content of the paper at someposition; h) measuring the concentration of filler or the concentrationof filler plus pub total concentration) in the white water or in thestock, preferably in a position in the short circulation or in directconnection therewith, characterised by adding the filler in an amountsuch that the system will be buffered with filler to a normallypredetermined concentration level (control value), said level beingfollowed via measurement (h); basing the continuing addition of fillerexclusively on the measured concentration level f filler in the whitewater or in the stock, such that when the measured level is lower thanthe control value, the addition of filler is increased, and such thatwhen said measured level is higher than the control value the additionof filler is reduced, at least in the long term, so that the white watersystem or the buffer system will always have available sufficient fillerto enable the paper web to adsorb the intended amount of filler; andbasing the continuing addition of retention agent exclusively on thenewly measured amount of filler in the paper (g), so that when themeasured amount of filler in the paper is lower than the level thatshall be held constant, the addition of retention agent is increased,and so that when the measured amount of filler in the paper is higherthan the level that shall be kept constant, the addition of retentionagent is reduced, therewith resulting in rapid correction of the fillercontent of the paper back to the level that shall be held constant.
 2. Amethod according to claim 1 characterised by adding one filler in one ormore positions.
 3. A method according to claim 1 characterised by addingthe total amount of filler to the stock in a position at the beginningof the short circulation.
 4. A method according to claims 1-3,characterised by slurrying the filler in a liquid prior to adding thefiller to the system, and by supplying said liquid and its fillercontent to the advancing pulp fibre suspension or to the water in saidposition with the aid of a regulator or several regulators operating inaccordance with described control philosophy.
 5. A method according toclaims 1-4, characterised by adding two retention agents to the systemin one or more positions.
 6. A method according to claims 1-4,characterised by adding two retention agents to the stock each in arespective position, wherein one retention agent is added at a pointlocated at a short distance into the short circulation in a variablequantity that is dependent on the amount of filler measured in the paper(g) at that moment in time, while the other retention agent is added ata point further forward in the short circulation, i.e. close to the headbox, in a constant amount based, for instance, on the paper productionand on the intended filler content of the paper.
 7. A method accordingto claims 1-6, characterised by slurrying and/or dissolving theretention agent in a liquid prior to its addition to the system, whereinliquid together with its retention agent content is added to theadvancing pulp fibre suspension or to the water in the describedposition with the aid of a regulator or several regulators operating inaccordance with the described control philosophy.
 8. A method accordingto claims 1-7, characterised by measuring the filler content of thepaper on the ultimately dried paper web having a dry solid content above90%.
 9. A method according to claims 1-8, characterised by measuring thefiller concentration of the white water in a white water sub-flow takenfrom the flow of white water transported from a collecting devicebeneath the wire section, said collecting device being designated a wiretray.
 10. A method according to claims 1-8, characterised by measuringthe filler concentration of the stock in a position immediately upstreamof the head box or inside said head box.